Closed circuit portable crushing and screening plants are utilized in quarries or at mining sites to process gravel or stone for commercial use and for crushing and screening other materials such as mineral ore including iron ore and the like. Conventional closed circuit portable crushing and screening plants include an upper conveyor which conveys material to a screening device which screens the material based upon size. The material which has been sufficiently crushed or which has attained the required size passes through the screening device and is discharged from the plant for commercial use or for further processing by another plant. The material which is too large passes from the screening device to a crusher. After being crushed, the material is discharged onto an underlying lower conveyor which conveys the material to an elevating wheel which elevates the material back up onto the upper conveyor for recirculation. The elevating wheel generally consists of a rotating drum with baffles mounted inside the drum. As the drum is turned slowly, the material is lifted and dropped onto the upper conveyor. Although frequently used in portable closed circuit crushing plants to elevate material from the lower conveyor to the upper conveyor, such elevating wheels are extremely expensive to manufacture due to their shape, are expensive to maintain and add energy costs to the system.
Due to the above disadvantages associated with the use of elevating wheels in portable closed circuit crushing plants, other elevating devices have been developed to lift the material from the lower conveyor to the upper conveyor. Couperus, U.S. Pat. No. 4,383,651 and Bronson, U.S. Pat. No. 4,598,875 disclose portable closed circuit crushing and screening plants which utilize an additional intermediate conveyor belt extending between the lower conveyor and the intermediate conveyor belt system having adjacent to elevate crushed materials from the lower conveyor to the upper conveyor. Each of the portable crushing and screening plants disclosed by U.S. Pat. Nos. 4,383,651 and 4,598,875 requires multiple pulleys or rollers to maintain the adjacent belts in sufficient contact with one another to sandwich and lift material between the belts. This intermediate conveyor system also requires an additional motor or other device for driving the adjacent belts. As a result, the intermediate conveyor belt system utilized by the portable crushing and screening plant is space consuming, complex and expensive.
Thus, there is a continuing need for a simple, easily manufactured, compact, lightweight, low-energy consuming, easily maintained and inexpensive elevating device for elevating at least partially crushed gravel, rock or minerals from a lower conveyor to an upper conveyor in a portable closed circuit crushing plant.